RESUMO
[reaction: see text] A high-yielding method is described for the rapid synthesis of very hindered cyclohexenes by the Diels-Alder reaction of hindered silyloxy dienes and dienophiles using the mixed Lewis acid catalyst system (AlBr(3)/AlMe(3)). Thus, reaction of the hindered diene 4 with various substituted enones gave good yields of the expected cycloadducts even though both partners are quite sterically hindered.
Assuntos
Alcenos/química , Cicloexanos/síntese química , Catálise , Ciclização , Cicloexanos/química , Cicloexenos , Estrutura MolecularRESUMO
[reaction: see text] A synthesis of the tricyclic [5-7-6] skeleton of guanacastepene A is described. The six-membered ring of guanacastepene A was constructed by a Diels-Alder reaction. After several functional group transformations, it was coupled to beta-iodocyclopentenone. Lithium dimethylcuprate conjugate addition followed by an intramolecular Mukaiyama aldol reaction furnished the desired [5-7-6] tricyclic ring system.
Assuntos
Diterpenos/síntese química , Cristalografia por Raios X , Indicadores e Reagentes , Modelos Moleculares , Conformação MolecularRESUMO
A liquid chromatographic method was developed for the determination of ciprofloxacin, enrofloxacin, and sarafloxacin at 10-200 ppb in both egg yolk and egg albumen of laying hens. Egg yolk or albumen was acidified with 1 M phosphoric acid followed by deproteination with acetonitrile and centrifugation. The supernate was pipetted out, and the remaining protein pellet was extracted three times with acetonitrile. The supernates were combined and concentrated at 50 degrees C to <0.7 mL. The final volume was adjusted to 2 mL with 0.02 M potassium phosphate buffer, pH 2.5. Separation of the analytes was achieved using reversed-phase HPLC with fluorometric detection. The recoveries were >80% and coefficients of variation <20%. After validation, the method was applied for use in a national survey for fluoroquinolones in table eggs. Of the 276 eggs assayed, none was found positive for fluoroquinolones. The findings suggest that illegal use of fluoroquinolones in laying hens is not widespread.
Assuntos
Galinhas , Cromatografia Líquida de Alta Pressão/métodos , Ciprofloxacina/análogos & derivados , Gema de Ovo/química , Fluorometria , Fluoroquinolonas/análise , Ovalbumina/análise , Drogas Veterinárias/análise , Animais , Anti-Infecciosos/análise , Ciprofloxacina/análise , Enrofloxacina , Feminino , Fluoroquinolonas/uso terapêutico , Doenças das Aves Domésticas/tratamento farmacológico , Controle de Qualidade , Quinolonas/análiseRESUMO
An efficient synthesis of the tetracyclic steroid core, 19, of rhodexin A and sarmentogenin is reported. An initial inverse-electron-demand Diels-Alder reaction of the acyldiene 6 with the silyl enol ether 7a gave the cycloadduct 8 with the required four contiguous stereocenters in a single step. This compound was then transformed into the methylated enedione 13 which afforded after a reductive alkylation and annulation sequence the tetracycle 19.